1. Field of the Invention
The present invention relates to a method for processing antenna patterns, more specifically, a method for two-dimensional beam compression processing which reduces antenna patterns two-dimensionally and a method for reducing a beam width as well as side lobes.
2. Description of the Related Art
A beam width and a side lobe are indices for describing the characteristics of antenna patterns of general antennas including receiving antennas. Narrower beam widths or lower side lobes give better antenna pattern characteristics.
However, the beam width shows an antinomical relationship with side lobes. Besides, the beam width is inversely proportional to an antenna size (length). That is, when the size of an antenna is constant, a reduction in the beam width results in an increase in side lobes and a reduction in side lobes leads to an increase in beam width. When the size of an antenna is modified, a reduction in beam width results in an increase in antenna size and a reduction in an antenna size results in an increase in beam width.
Therefore, for example, in radar antennas, regarding a relationship between a beam width and a side lobe, when side lobes are reduced, then the beam width is widened and the resolution or the ability to discriminate targets gets degraded. As a result, the radar could misidentify plural targets as single target. On the other hand, when the beam width is reduced, side lobes become high and the radar might misidentify a target which exists in the direction of certain side lobes even though it exists in the observation direction, although there is nothing in the observation direction. As for a relationship between a beam width and an antenna size, when the beam width is reduced by half in order to increase the ability to discriminate the target by a factor of two, then the antenna size necessarily becomes twice as large. Such an increase in size leads to various kinds of problems, such as an increase in occupied space, an increase in the weight of the antenna, and an increase in the size of the structure for supporting the antenna. On the other hand, a reduction in the size of the antenna by half results in a two-fold increase in the beam width by twice, thus decreasing the discrimination ability by half.
As described above, the beam width and the side lobes have characteristics conflicting with each other, hence it is impossible to optimize both of these at the same time. Therefore a compromise is required between a beam width and side lobes. A possible compromise has been to take a certain distribution such as Chebyshev distribution so that the beam width is minimized whenever a certain condition exists regarding the side lobes, or side lobes are minimized regarding a certain condition of a beam width. On the other hand, because there is a conflicting relation between the beam width and the antenna size, as shown above, and moreover, because there is a limitation in space for the antenna to occupy in most actual cases, a compromise has been made so that a moderate beam width is obtained under these restrictions.
As for a method to partially solve such problems, a multiplicative array is known as a method for reducing beam widths by multiplying each signal with each other received with plural antennas. FIG. 1 shows the constitution of such an antenna which reduces beam widths. In this figure, 101 is a main antenna such as an array antenna comprising plural radiating elements arranged along a straight line with equal spaces and 102 is a reference antenna. The reference antenna 102 is arranged apart from the main antenna 101 in the X direction which is the orientation of the beam width to be reduced. 103 is a multiplier which multiplies the signal received with the main antenna 101 by the signal received with the reference antenna 102. In an antenna systems having such a constitution, each signal received with antennas 101 and 102 is applied to the multiplier 103 with the same phase to perform the multiplication between the directional characteristic of the main antenna 101 and that of the reference antenna so that the resulting synthesized directional characteristic shows a reduction in the beam width.
In the above conventional method for reducing the beam width, the reference antenna 102 is arranged to adjoin the main antenna 101 in only one direction. Therefore, only a one-dimensional reduction in the beam width is possible and, as a result, the discrimination ability is inadequate. In the case that a main beam has such a pattern 201 shown in FIG. 2A, which has no reduction in a beam width, it is impossible to discriminate five targets a, b, c, d, and e each other. When this pattern is reduced one-dimensionally in the X direction, then the resultant reduced pattern becomes such a pattern 202 shown in FIG. 2B. Even in this case, while the ability of the discrimination is improved compared to the case with no reduction in a beam width, there is still a problem that it is impossible to discriminate three targets a, c, and e each other, thus the discrimination ability is inadequate.
Furthermore, it should be noted that the above conventional method for reducing the beam width has the problem that only a one-dimensional reduction in the beam width is possible and it is impossible to reduce side lobes in addition to the beam width at the same time because the reference antenna 102 is arranged to adjoin the main antenna 101 only in one direction.